In present-day telephone systems, the subscriber stations are generally energized by a central battery connected across the talking conductors of their respective line loops whereby, upon closure of a line loop by the subscriber's hook switch, a direct current circulates over these conductors to mark the line busy and to convey switching signals such as dial pulses. The switching equipment responding to these signals at the central office is connected to the talking conductors, at points lying on the subscriber side of respective blocking capacitors or transformers, by circuit elements having a low resistance to direct current but a high impedance for alternating current in order to prevent the transmission of voice frequencies between lines supplied by the same battery. The high a-c impedance is normally an inductance which in many instances is constituted by the winding of an associated busy relay.
In contrast to resistances and capacitances, an inductance is not readily realizable in integrated circuitry and must therefore generally be designed as a coil which, aside from occupying considerable space, has to be protected against dust and moisture; such coils, moreover, are relatively expensive on account of the considerable quantity of copper required for them.
Efforts have therefore already been made to replace the conventional line inductances of telephone systems by electronic components of more or less equivalent characteristics. Thus, the use of constant-current generators has been proposed (Zurich Seminar 1976 and 1978, Paper Co. C7) as a means for suppressing current alternations and providing a direct current of a magnitude nearly independent of line length. Drawbacks of this method include the difficulty of obtaining a perfect balance between the current generators connected to the two line conductors as well as their incompatibility with variable-gain electronic telephones in which the intensity of voice-frequency currents is adjusted in response to direct-current magnitude to compensate for differences in line length.
According to another proposal (Zurich Seminar 1976, Paper No. C8), inductances are simulated by integrated circuits comprising two resistors in series. This circuitry is rather complex and consumes considerable power (about 0.6 watt). It also requires the insertion of a small transformer in the line loop to reduce interference by spurious signals of the so-called common-mode type which propagate cophasally along the two line conductor to their common ground return but which are not transmitted to the secondary of that transformer.
A transistor circuit has also been described (Zurich Seminar 1976, Paper No. C7) which simulates a busy relay as concerns response to line current and inductive behavior. Such a circuit, however, has a nonlinear voltage/current characteristic especially in the low-voltage range and is sensitive to current reductions and noise pulses which may be misinterpreted as switching signals.